1. Field of the Invention
The invention pertains to a mounting for an array of crystal detector elements for pyroelectric detectors, particularly those made from crystalline lithium tantalate.
2. Description of the Prior Art
Crystalline lithium tantalate (LTO) is widely used in pyroelectric detectors and detector arrays. Pyroelectric detectors are thermal detectors which respond to a temperature change by producing an electric charge which is measured as a signal voltage via interfacing electronics. The detector element is typically comprised of a thin (20 to 200 micron thick) slab of crystal material which has been processed to be polarized and then ground, polished and patterned to produce individual pixels or arrays. Typical element sizes used are 0.5 mm square linear arrays of n such pixels (n=4, 8, 16 etc.) which may be separated by 0.1 mm spacing. Other sizes and configurations are clearly possible since the pyroelectric detector is a capacitive structure.
A typical use of such a detector is in satellite guidance such as is disclosed in U.S. Pat. No. 5,055,689 to Proffit and Rose entitled "Horizon Sensor Apparatus and Method Therefor"; U.S. application Ser. No. 08/229,460 filed Apr. 18, 1994 entitled "Earth Sensor for Satellites with Radiance Compensation"; and U.S. application Ser. No. 08/227,270 filed Apr. 13, 1994 entitled "Earth Sensor for Satellites", now U.S. Pat. No. 5,455,424 .
The electro-optical performance of these pyroelectric detectors and detector arrays is largely determined by the crystal size and shape, and the means used to mount the crystal. In order to maximize the voltage output, a large thermal time constant of several hundred milliseconds is required. For this, the crystal needs to be supported as minimally as possible in order to reduce heat drain from the crystal and provide thermal isolation. In prior art structures, the crystal elements have been supported by wire loops or a ceramic or plastic frame attached around the crystal perimeter with epoxy. Problems are associated with these schemes of crystal mounting. Firstly, the pyroelectric crystals are also piezoelectric. The mounting scheme must therefore minimize vibrations transferred to the crystal to obtain low microphonic output. Secondly, the detectors often need to operate over a large range of operating temperatures. The mounting structure must minimize the thermal stresses generated at the crystal by differential expansions of the support and crystal as the thermal stresses can generate voltage spikes (i.e., "glitches") from the device.